Within the IP (Internet Protocol) Multimedia Subsystem (IMS) as defined by 3rd Generation Partnership Project (3GPP) Session Initiation Protocol (SIP) defined by Internet Engineering Task Force (IETF) is used for controlling communication. SIP is an application-layer control protocol for creating, modifying, and terminating sessions with one or more participants. These sessions may include Internet multimedia conferences, Internet telephone calls, and multimedia distribution. Members in a session can communicate via multicast or via a mesh of unicast relations, or a combination of these. Forking is ability of a SIP proxy to send SIP requests to multiple destinations at the same time.
The standardized IMS includes number of features that have relationships with each other. One of features is capability to share the same user identity in multiple devices as shown in this FIG. 1. For example, a user can have two or more devices that share the same identity for example the same IMS public user identity (IMPU), like public user identity 2 in FIG. 1. The shared identity can be used to initiate communication with any of these devices. Similarly a user can receive communication targeted to this identity with any of the devices bound to this identity. Identity concept could be extended also to cover use case where two or more different users, for example two family members, are sharing the same identity.
Voice call continuity (VCC) as defined by the 3GPP defines a capability to offer mobile voice services for a multi-radio terminal user via both circuit switched (CS) and packet switched (PS)/IMS access as preferred. The feature also contains capability to transfer ongoing call from one access to another e.g. from wireless local are network (WLAN) to CS access.
Single Radio VCC (SR-VCC) enables to perform handover from long term evolution (LTE) radio access network to CS access network. A network function is acting as a gateway between UE and an application server (AS) providing voice call continuity feature. The network function which can also be called as interworking function (IWF) can initiate a handover request towards the application server (AS) providing the voice call continuity based on received information from a radio access network, such as LTE, and from an evolved packet core system.
Multimedia session continuity enables continuity of PS media streams when PS access is changed, for example, by transferring a video session from WLAN to Universal Mobile Telecommunications System (UMTS) terrestrial radio access network (UTRAN) using IMS and application layer mechanisms. IMS centralized services (ICS) provide service execution in the IMS even though a device can use CS network to make calls. All services are executed in the IMS and all originated and terminated CS calls are forcedly routed to the IMS for service execution. To manage mid-call services ICS UE must use SIP signalling to initiate service execution. This control channel is called as I1-PS in ongoing standardization.
It is possible an IMS user to register its Public User Identity from several devices. In addition, the user may have multiple CS capable devices under his IMS Public User Identity. In this case parallel ringing can take place. Currently there exist challenges in call termination if the user has the same device reachable via both CS and PS access at the same time and forking is used to implement parallel ringing.